Multi-Compressor Refrigeration System and Method for Operating It

ABSTRACT

A refrigeration system ( 20 ) has a first compressor ( 24 ) and a second compressor ( 26 ). The second compressor has at least a first condition t least partially in parallel with the first compressor along a refrigerant flowpath. A heat rejection heat exchanger ( 50 ) is downstream of the first and second compressors along the refrigerant flowpath. An expansion device ( 54 ) is downstream of the heat rejection heat exchanger along the refrigerant flowpath. A heat absorption heat exchanger ( 56 ) is downstream of the expansion device along the refrigerant flowpath. The first compressor is a variable speed compressor coupled to a variable speed drive ( 32 ). The second compressor is a fixed speed compressor.

CROSS-REFERENCE TO RELATED APPLICATION

Benefit is claimed of U.S. patent application Ser. No. 61/486,496, filedMay 16, 2011, and entitled “Multi-Compressor Refrigeration System”, thedisclosure of which is incorporated by reference herein in its entiretyas if set forth at length.

BACKGROUND

The disclosure relates to refrigeration. More particularly, thedisclosure relates to refrigerated transport containers using CO₂-basedrefrigerant.

CO₂-based refrigerant such as R744 has drawn increasing attention foruse in refrigerated transport containers. Exemplary refrigeratedtransport containers include shipping containers and containers integralwith trucks, trailers, or rail cars. Such containers, especiallyshipping containers, may be subject to a wide variety of operatingconditions. The operating conditions reflect both theexternal/environmental temperature and the interior temperature.Interior temperature varies based upon the nature of the goods beingtransported, with low temperatures being required for frozen goods andhigher temperatures being required for non-frozen refrigeratedperishable goods. Exemplary systems include an electrically-poweredcompressor for driving refrigerant along a circuit/flowpath through anexterior heat rejection heat exchanger and an interior heat absorptionheat exchanger.

SUMMARY

One aspect of the disclosure involves a refrigeration system having afirst compressor and a second compressor. The second compressor has atleast a first condition at least partially in parallel with the firstcompressor along a refrigerant flowpath. A heat rejection heat exchangeris downstream of the first and second compressors along the refrigerantflowpath. An expansion device is downstream of the heat rejection heatexchanger along the refrigerant flowpath. A heat absorption heatexchanger is downstream of the expansion device along the refrigerantflowpath. The first compressor is a variable speed compressor coupled toa variable speed drive. The second compressor is a fixed speedcompressor.

In various implementations, the fixed speed compressor may have a largerdisplacement than the variable speed compressor. The compressors may bereciprocating compressors. This may be in an operational condition withthe fixed speed compressor connected directly to a line voltage and thevariable speed compressor connected to the line voltage via its variablespeed drive.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a refrigeration system.

FIG. 2 is a view of a refrigerated container.

FIG. 3 is a control flowchart for the system of FIG. 1.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a vapor compression system 20 having a compressor subsystem22. The exemplary compressor subsystem 22 includes a first compressor 24and a second compressor 26 at least partially in parallel with the firstcompressor along a refrigerant flowpath 500. The exemplary first andsecond compressors are both reciprocating compressors. The exemplarycompressors have respective electric motors 28 and 30. The exemplarymotor 28 is powered by a variable frequency drive (variable speed drive(VSD)) 32 in turn powered by line power 34. The exemplary motor 30 isdirectly powered by the line power 34. Exemplary line power is 50 Hz or60 Hz providing three-phase power to the motor 30 and VSD 32. Forexample, in the case of a cargo container drawing line power from agenerator on the ship carrying the container, the exemplary voltage is460V.

The exemplary motors 28 and 30 are hermetic induction motors. Analternate motor 28 is a permanent magnet motor. The exemplary motor 28is rated for adjustable speed duty (ASD) under applicable industrystandards. An ASD-rated motor will likely have more robust windinginsulation than a non-ASD-rated motor. The line voltage may represent amaximum of the VSD output voltage, but the VSD may be configured toprovide a higher than line frequency at such maximum voltage. Theexemplary VSD is capable of running the motor 28 across a frequencyrange spanning the line frequency. An exemplary low end of the range is15-20 Hz. An exemplary high end of the range is at least 110 Hz or 120HZ. For example, with an exemplary line power of 460 V and 60 Hz, at aVSD output frequency of 120 Hz, the VSD output voltage may be thatmaximum 460 V. With a linear V/f curve, the VSD output voltage is 230 V@60 Hz and 77V@20 Hz (an exemplary lowest operating frequency of themotor 28 noted above and further discussed below).

Downstream from the compressor discharge ports, the refrigerant flowpathsequentially passes through a first heat exchanger 50, a receiver 52, anexpansion device 54, and a second heat exchanger 56. In the first mode,the heat exchanger 50 is a heat rejection heat exchanger (e.g., a gascooler or condenser) and the second heat exchanger 56 is a heatabsorption heat exchanger (e.g., an evaporator). The exemplary heatexchangers 50 and 56 are refrigerant-air heat exchangers whereinelectric-powered fans 60 and 62, respectively, drive air flows 64 and 66across the transfer elements (e.g., coils).

A controller 70 may be coupled to various controllable system components(e.g., the compressor motors, the fans, the expansion device or anyother control valves, and the like). The controller may be coupled toreceive inputs from sensors (e.g., pressure and/or temperature sensors).Exemplary sensors include a supply air sensor 72 positioned to measurethe temperature of the flow 66 exiting the heat exchanger 56, a returnair temperature sensor 74 positioned to measure the temperature of theair 66 entering the heat exchanger 56, and an ambient/externaltemperature sensor 76 (e.g., positioned to measure ambient/external airtemperature of the flow 64 entering the heat exchanger 50).

The exemplary system 20 is used in a refrigerated transport system 200(FIG. 2). An exemplary system is shown as a shipping container 201having a refrigerated compartment 202. An equipment compartment 204 islocated at one end of the container and contains the components of thesystem 20. The evaporator 56 in the refrigerated compartment 202 (or inair flow communication with the refrigerated compartment 202 via therecirculating air flow 66). Other similar refrigerated transport systemsinclude trucks and trailers as described above. The exemplary shippingcontainer draws power from an external source (e.g., the generator of aship). However, truck and trailer systems are more likely to includeelectrical generators (e.g., diesel engine-powered electricalgenerators). Such electrical generators may be in housings external tothe main box of the truck or trailer (e.g., along with the compressorand heat rejection heat exchanger).

In configuring the system, total cost of ownership (TCO) is an importantconsideration. Common industry practices have arisen involving measuringcost at specific operating conditions (the TCO points) which may beassociated with specific users (e.g., at which such users spend majorityof time). Each TCO point is characterized by an ambient temperature anda refrigerated compartment temperature. If a single compressor were usedand sized to meet the full load pulldown capacity requirement, it wouldhave substantial extra capacity at partial load conditions which providethe majority of TCO points. Such excess capacity would involveinefficient operation. Accordingly, the presence of multiple compressorsmay allow sizing to provide better efficiency at the lower load TCOpoints and meet the full load pulldown capacity requirement (even if useof multiple compressors provides lower efficiency during pulldown).

In an exemplary implementation, the variable speed compressor is runalone over a load range from minimal load to an intermediate load. Thevariable speed compressor is thus sized to provide maximum efficiencyover this range which includes the majority of TCO points. Once thevariable speed compressor has been sized, the fixed speed compressor maybe sized to make up for the difference between the maximum capacity ofthe variable speed compressor and the maximum required system capacity.The maximum required system capacity is usually defined by an extreme ofanticipated need to provide cool down (pulldown) in an initial operatingcondition. Steady state operating condition is typically at asubstantially lower capacity. The exemplary fixed speed compressor islarger than the exemplary variable speed compressor. An exemplary sizemeasurement is displacement per revolution. The exemplary fixed speedcompressor is 110 to 350% the size of the variable speed compressor,more narrowly, 125-350%. Yet, more narrowly, 125-250%. Even though thevariable speed compressor has a smaller displacement, the ability of theVSD to output frequencies greater than line frequency allows the smallerdisplacement variable speed compressor to be run at capacities which mayexceed the capacity of the fixed speed compressor run at line power.This allows the variable speed compressor to be operated alone at lowloads without having a gap in load handling. More particularly, the peakcapacity of the variable speed compressor may meet or exceed thecombination of the capacity of the fixed speed compressor and theminimum capacity of the variable speed compressor. This allows a smoothhandover between operations in: a mode where only the variable speedcompressor runs; and a mode where both compressors run (without a gapbetween available capacities of those two modes).

FIG. 3 shows a basic control algorithm 300. There is a start-up 302. Therelationship between a measured or otherwise determined systemtemperature and a target temperature is then determined. For example,the measured temperature may be the temperature T_(C(S)) of the supplyair measured by the sensor 72 or T_(C(R)) of the return air measured bythe sensor 74. The desired temperature may be a user-entered temperatureset point. The relationship may involve determining 304 whether themeasured temperature exceeds the set point by at least a given threshold(DT). Exemplary DT may be preset based upon the nature of the use (e.g.,refrigerating frozen goods versus refrigerating non-frozen perishablegoods) and the particular measured temperature may be determined by suchuse. For example, in a frozen goods scenario, a main focus of operationmay be to avoid temperature high enough to melt the goods. The returntemperature T_(C(R)) may be measured to ensure that it does not exceed auser-entered temperature setpoint. An exemplary target returntemperature may be relatively low (e.g., less than 14.4 F). Fornon-frozen perishable goods, it may be more important to measure thesupply temperature to ensure that the supply temperature is not so lowas to freeze the goods that the air initially comes in contact with.Exemplary supply temperature is thus in excess of 14.4 F. For frozengoods, there may be more flexibility in DT than for non-frozenperishable goods. Thus, an exemplary DT for frozen goods isapproximately 4 F whereas a DT for non-frozen perishable goods isapproximately 0.5 F. This effectively determines whether the system isin a high capacity situation or a low capacity situation. If yes (highcapacity situation), then the compressors are run simultaneously 310with the variable speed compressor at a temperature-dependent speed upto its maximum speed. This provides the fastest possible cool down/pulldown.

If not in the high capacity situation, then it is determined 312 whetherany cooling is required (e.g., the measured temperature is greater thanthe set temperature by less than the DT) (low capacity situation). Ifno, then both compressors are shut put in their off conditions 314 andthe cycle can repeat. If yes, then it is determined 316 whether thefixed speed compressor has been off or it has run for its minimum time.This determination 316 helps avoid short cycling of the fixed speedcompressor. If yes at 316, then only the variable speed compressor isrun 318. It is run at a speed appropriate for the needed capacity. Ifno, then there is simultaneous operation 310 to avoid short cycling ofthe fixed speed compressor. If the variable speed compressor only is runat 318, then it is determined 320 whether it has been at its maximumspeed for a predetermined time. Yes indicates that the variable speedcompressor alone is not effective to bring down the temperature quicklyenough. Thus, if yes, then simultaneous operation is resumed at 310. Ifno, however, there is a minimum capacity which the variable speedcompressor may provide in continuous operation. It is thereforedetermined 322 whether required speed has reduced to this minimum speedfor a predetermined time. If no, the control cycle merely repeats atstep 304. If yes, then the variable speed compressor is shut off at 314.

Once in simultaneous operation at 310, there is then a determination 330of whether the variable speed compressor is being operated within acertain proximity of its minimum speed. if no, then the cycle repeats at304. If yes, then the system shifts to variable speed compressor onlyoperation at 318.

In the foregoing example, the controller is configured to in no part ofa normal operational range operate the fixed speed compressor alone.However, this may be done in abnormal situations such as a failure ofthe variable speed compressor or associated components, a service mode,or a manual override mode where the user commands shut down of thevariable speed compressor.

Although an embodiment is described above in detail, such description isnot intended for limiting the scope of the present disclosure. It willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, when appliedto the reengineering of the configuration of an existing system, detailsof the existing system may influence details of any particularimplementation. Accordingly, other embodiments are within the scope ofthe following claims.

1. A refrigeration system (20) comprising: a first compressor (24); asecond compressor (26) having at least a first condition at leastpartially in parallel with the first compressor along a refrigerantflowpath; a heat rejection heat exchanger (50) downstream of the firstcompressor and second compressor along the refrigerant flowpath; anexpansion device (54) downstream of the heat rejection heat exchangeralong the refrigerant flowpath; and a heat absorption heat exchanger(56) downstream of the expansion device along the refrigerant flowpath,wherein: the first compressor is a variable speed compressor coupled toa variable speed drive (32) and the second compressor is a fixed speedcompressor; and the second compressor is larger than the firstcompressor.
 2. (canceled)
 3. The system of claim 1 wherein: the secondcompressor has a larger displacement per revolution than a displacementper revolution of the first compressor.
 4. The system of claim 1wherein: the first compressor and the second compressor arereciprocating compressors.
 5. The system of claim 1 in operationalcondition with the second compressor connected directly to a linevoltage (34) and the first compressor connected to the line voltage viaits variable speed drive.
 6. A transport system (200) comprising: therefrigeration system (20) of claim 1; and a refrigerated container (201)having an interior (202) containing or in air flow communication withthe heat absorption heat exchanger.
 7. (canceled)
 8. The system of claim1 wherein: a displacement per revolution of the second compressor is110-350% of a displacement per revolution of the first compressor. 9.The system of claim 1 wherein: the first compressor has an inductionmotor or a permanent magnet motor; and the second compressor has aninduction motor.
 10. The system of claim 1 further comprising acontroller configured to: at high required capacity (upper range),operate (310) both the first compressor and the second compressor, thesecond compressor being operated at a fixed speed; and in low requiredcapacity (lower range), operate (318) only the first compressor, over atleast a portion of said lower capacity range the operating being withvariable speed.
 11. The system of claim 10 wherein the controller isconfigured to in no part of a normal operational range operate thesecond compressor alone.
 12. A method for operating the system of claim1, the method comprising: at high required capacity (upper range),operating (310) both the first compressor and the second compressor, thesecond compressor being operated at a fixed speed; and in low requiredcapacity (lower range), operating (318) only the first compressor, overat least a portion of said lower capacity range the operating being withvariable speed.
 13. The method of claim 12 wherein: the lower rangemeets the upper range.
 14. (canceled)
 15. The method of claim 12wherein: the operation of the first compressor in an uppermost portionof the lower capacity range is at a power frequency in excess of a linepower frequency.
 16. The method of claim 12 wherein: a cooldown phasecomprises the operation in the high capacity range; and a post-cooldownphase comprises the operation in the lower capacity range.
 17. Themethod of claim 12 wherein: the control is responsive to a sensed airtemperature of a controlled space.
 18. A method for operating arefrigeration system, the refrigeration system comprising: a firstcompressor (24); a second compressor (26) having at least a firstcondition at least partially in parallel with the first compressor alonga refrigerant flowpath; a heat rejection heat exchanger (50) downstreamof the first compressor and second compressor along the refrigerantflowpath; an expansion device (54) downstream of the heat rejection heatexchanger along the refrigerant flowpath; and a heat absorption heatexchanger (56) downstream of the expansion device along the refrigerantflowpath, wherein: the first compressor is a variable speed compressorcoupled to a variable speed drive (32) and the second compressor is afixed speed compressor, the method comprising: controlling operation ofthe compressor responsive to a sensed air temperature of the controlledspace.
 19. The method of claim 18 wherein: the sensed air temperature isused to control transitions between operation of only one of thecompressors and both of the compressors.
 20. A method for operating arefrigeration system, the refrigeration system comprising: a firstcompressor (24); a second compressor (26) having at least a firstcondition at least partially in parallel with the first compressor alonga refrigerant flowpath; a heat rejection heat exchanger (50) downstreamof the first compressor and second compressor along the refrigerantflowpath; an expansion device (54) downstream of the heat rejection heatexchanger along the refrigerant flowpath; and a heat absorption heatexchanger (56) downstream of the expansion device along the refrigerantflowpath, wherein: the first compressor is a variable speed compressorcoupled to a variable speed drive (32) and the second compressor is afixed speed compressor, the method comprising: at high required capacity(upper range), operating (310) both the first compressor and the secondcompressor, the second compressor being operated at a fixed speed; andin low required capacity (lower range), operating (318) only the firstcompressor, over at least a portion of said lower capacity range theoperating being with variable speed, wherein: the lower range comprisesa lower sub-range wherein the first compressor is operated in a cyclicmode with essentially fixed speed when operating and, an upper sub-rangeoperated continuously with speed increasing with required capacity. 21.The method of claim 20 wherein: the lower range meets the upper range.22. The method of claim 20 wherein: the operation of the firstcompressor in an uppermost portion of the lower capacity range is at apower frequency in excess of a line power frequency.
 23. The method ofclaim 20 wherein: a cooldown phase comprises the operation in the highcapacity range; and a post-cooldown phase comprises the operation in thelower capacity range.